Want to know how to change the Nib and Refill the ink on a Rotring Isograph Pen. look no further as full step by step instructions are here! The difference between the Isograph Pen and the Rapidograph pens is how you refill the ink. The Rapidograph pen uses a cartridge whereas the Isograph pen fills with a bottle of ink. Unfortunately Rotring are discontinuing the Rapidograph pens although we still have the ink cartriges for sale. Check out the video on how to change the Isograph nibs below.